With the introduction and the development of optical networks it is a goal to reduce the cost and complexity of data transmission within voice and data networks. A major factor for achieving this is to reduce the number of signal transformations between optical and electrical signals. Such a reduction will reduce the number of components within the networks elements and reduce the need for electronic signal processing. Further a reduction in the number of components within the networks element will result in a reduction of the sources of errors, and hence reduced need for service and maintenance and an increased operational time. These factors will again result in a potentially reduced cost.
The traffic volume of Internet is reported to show a significant increase despite the downturn of the telecommunication industry. Hence, ingreasing parts of the traffic in the transport network origins from packet data. For obvious economic reasons, new switching techniques should first be introduced at the time they show maturity and cost effectiveness. Hence there is a need to develop flexible optical networks supporting a seamless migration from an optical circuit switched (OCS) to an optical packet switched (OPS) backbone network.
Thus replacing electronical network element with optical network elements it is necessary that the optical network elements have a functionality which can operate effectively within a packet switched network. In the last few years intensive research have been spent on optical packet switching (OPS), and optical burst switching where packets or bursts of packets are switched directly in the optical layer with optical switches. These techniques are expected to be commercially of interest within approximately four years.
The Five Dimensions
As optical signal processing is still immature there are very restricted possibilities for signalling different types of information such as address information. Dimensions available for transfer of information in an optical fibre are: intensity, time, frequency, phase and polarization. All these dimensions are through the years suggested used for different purposes.
The formats of modulation used in optical links and networks are today based on NRZ- and RZ-formats where intensity varies between a minimum- and maximum level. The signals are time divisional multiplexed (TDM) with a data rate between 2.5 and 40 Gb/sec. In optical line switched networks the available and useable optical frequency spectrum is used for multiplexing a number of TDM-channels within one fibre, so called Wavelength Division Multiplexing (WDM). The optical frequency is also suggested used as a label with optical networks where the framework from MPLS is used. Phase and frequency are suggested used as a form of modulation as to increase spectral efficiency likely in combination with polarization.
Optical Package Switching, Address, QoS and Signalling.
In connection with optical package switching transfer of address information in the form of a header or a label is a problem for discussion. Normally, in an electronic router the header will be transferred at the beginning of the package or the frame, and the address information and payload is thereby time multiplexed. Demultiplexing in the time domain is difficult using optical components. Transfer of address information separated from payload is therefore suggested carried out in different manners such as:
1a) Address and payload are separated by the use of separate optical wavelengths; this gives however a bad utilization of the wavelengths.
1b) Usage of a separate frequency within the optical wavelength, so-called Sub Carrier Modulation (SCM), utilizing the optical wavelength more efficiently than when a separate wavelength is used. However, this solution may lead to a deterioration of the payload signal.
1c) In the EU-sponsored project “STOLAS” it is suggested to use frequency modulation for modulation of package header separated from the payload; however this method may also give a deterioration of the signal quality within the payload. STOLAS is an ongoing project within EUs 5th general plan “IST”. Reference for this theme within the project: Sulur, T. K. et al. “IM/FSK Format for Payload/orthogonal Labelling IP Packets in IP over WDM Networks Supported by GMPLS-based LOBS.” ONDM 2003, Feb. 3-5, 2003, Budapest, Hungary.
Several techniques have been proposed for in-band header encoding, like serial header, SubCarrier Modulation (SCM), and Frequency Shift Keying (FSK). However, they require advanced components for separation of header and payload, and reinsertion of new headers. To erase old header, before a new one can be inserted, per input wavelength, serial header requires a fast optical gate e.g. a Semiconductor Optical Amplifier (SOA), while SCM and FSK need an optical wavelength converter. This increases component count of complex and yet technologically immature components. Furthermore, if separation of packets belonging to different QoS classes is desirable, it will normally be done based on electronic processing of the header information, hence not all-optical.
Known Principles
1) Use of polarization of multiplexing/demultiplexing of two data channels (multiplexing by polarization) on one fibre is a known principle.
2) Use of polarization to find a start and stop on a bit-sequence is known, consequently by changing state of polarization.
3) To separate different optical data channels by polarization in the same manner as different optical data channels may be separated on its wavelength. Optical ad/drop entities based on separation between orthogonal polarizations like similar entities which are distinguished by the use of wavelength demonstrated and referred to in the literature.
4) Separation of header- and payload by the use of polarization is patented.
Introduction
In a statistical multiplexed packet switched network, services like constant delay, and no packet loss, can not be guaranteed due to the very nature of statistical multiplexing. This may preclude the use of strict real-time applications, where delay is critical, and packet loss should be at an absolute minimum, like e.g. for remotely controlled surgery. Guaranteed service (GS), without contention causing packet loss, and a fixed delay, can however be offered if the packets are sent through a network following a path with pre-assigned resources, like in a Static or Dynamic Wavelength Routed Optical Network (S-WRON or D-WRON). D-WRONs increases throughput efficiency, compared to S-WRONs, by dynamically reconfiguring the wavelength paths to adapt to the traffic demands. However, the control plane operates on an ms to s timescale, and cannot be optimized to the bursty traffic patterns of OPS, where packet durations are typically in the μs range. Therefore, not even D-WRONs can achieve the throughput efficiency and granularity of statistical multiplexing.
The Package Switch
A package switch may be partial optical and partial electronic or fully optical.
In EP 07944684 A1 it is described an optical package switched network with one or several nodes and a transmitter sending polarized package signals. The package signals comprising a header- and a payload separated from each other by way of orthogonal polarizing. Further it is known from CA 2352113 an optical method of communication where it is utilized a high speed polarized bit stuffing method. The method describes a way of using polarized bit-stuffing for separation of data package instead of multiplexing data streams from different modulators. This increases the speed for transferring of data within an optical network.
Optical packet switching (OPS) is promoted as a way to overcome the electronic bandwidth bottleneck. However, if OPS nodes are to be realised, they must also prove to be cost effective. The present invention proposes to use polarisation multiplexing for a low-cost separation and reinsertion of control information in OPS, as well as for optical differentiation between Quality of Service (QoS) classes. The two applications can be performed simultaneously or separately.
In the present invention it is proposed to combine the properties of a statistically multiplexed packet switched network (OPS) with the GS enabled by optical circuit switched networks (like S-WRON/D-WRON) in a single optical network layer. This requires that the circuit switched GS packets and the OPS packets efficiently share the data layer resources. A node design that allows full sharing of link bandwidth is proposed, and that allows a migration from an S-WRON to the more efficient combined network, by adding OPS capability. The efficiency of the node is studied using a simulator.
The technique proposed here, as presented in the present invention, overcomes the drawbacks as described above by using orthogonal States of Polarisation (SOP) for separating packets and sending control information. By using a Polarisation Beam Splitter (PBS) per wavelength for header/payload separation, the complexity and cost may be reduced significantly, compared to the solutions mentioned above.